1. Field of the Invention
The present invention relates to magnetoresistive sensors for reading magnetically-recorded information from data storage media, and particularly to MTJ (Magnetic Tunnel Junction and CPP (Current-Perpendicular-to-Plane) spin valve read sensors for direct access storage device (DASD) systems.
2. Description of the Prior Art
By way of background, MTJ and CPP spin valve devices have been proposed for use as magnetic field sensors (e.g., read head sensors) in DASD systems, such as disk drives. An MTJ sensor is a variable current device formed by a magnetically pinned ferromagnetic layer and a magnetically free ferromagnetic layer separated by an intervening tunnel barrier layer. The barrier layer is made from a material that is electrically insulating but thin enough to pass charge carriers via a quantum mechanical phenomenon known as electron spin tunneling. Electrical leads and/or shields are positioned to make electrical contact with the ferromagnetic layers and induce a sense current that passes perpendicularly through the tunnel barrier layer from the pinned ferromagnetic layer to the free layer. Because the tunneling phenomenon is electron-spin dependent, the electrical resistance across the barrier layer is related to the spin polarity of the tunneling current, which in turn is a function of the alignment of the magnetic moment (magnetization directions) of the free layer relative to that of the pinned layer. The electron tunneling probability is highest when the magnetic moments are parallel and lowest when the magnetic moments are antiparallel. Because the magnetic moment of the free layer can be influenced by an external magnetic field, such as a magnetic domain on a data storage medium, an MTJ sensor""s electrical resistance will vary as a function of the direction and magnitude of the recorded magnetic domains, which rotate the free layer""s magnetic moment with respect to the relatively fixed magnetic moment of the pinned layer.
A CPP spin valve sensor is generally similar in construction to an MTJ sensor except that the tunnel barrier layer is replaced with an electrically conductive barrier layer. The CPP spin valve sensor thus comprises a stacked array of a pinned ferromagnetic layer, an electrically conductive metallic barrier layer, and a free ferromagnetic layer, sandwiched between a pair of electrical leads and/or shields. Like an MTJ sensor, electrical current in a CPP spin valve sensor is passed perpendicularly to the planes of the stacked array layers, and electrical resistance is measured. The sensor readback signal is a function of the resistance changes that result when the magnetic moment of the free layer rotates relative to the pinned layer magnetic moment under the influence of recorded magnetic domains. Resistance is lower when the relative magnetic moments are parallel and higher when the magnetic moments are antiparallel. Because of their superior sensitivity, CPP spin valves are generally preferred over CIP (Current-In-Plane) spin valves.
Commonly assigned U.S. Pat. No. 5,898,547 of Fontana et al. (the ""547 patent), discloses a magnetoresistive read head (with an MTJ sensing element) for use in an integrated read/write head of a disk drive. The sensing element of the read head is recessed from the sensing surface of the head to minimize damage to the sensing element layers during lapping. A flux guide is formed as an extension of the sensing element free layer and carries magnetic flux from the sensing surface to the sensing element.
A disadvantage of the read head of the ""547 patent is that the flux guide and the sensing element structures are defined by separate photo processing operations, and the flux guide has a track width which is narrower than that of the sensing element. It is believed that this reduces read sensitivity because the magnetic flux carried from the narrower track width of the flux guide into the wider track width of the sensing element device becomes diluted.
Another disadvantage of the read head of the ""547 patent stems from the use of hard biasing material to longitudinally bias the sensing element free layer for noise reduction and improvement of signal-to-noise characteristics. In the disclosed read head, the direction of longitudinal biasing is parallel to the sensing surface and across the track width of the free layer. The hard biasing material is disposed on either side of the track width boundaries of the free layer, but because it is electrically conductive, the biasing material must be spaced from the sensing element free layer and separated therefrom by an insulator. In order to have good insulating quality, the insulator must be relatively thick. Unfortunately, the required insulating thickness is sufficiently large that hard biasing effectiveness relative to the free layer is lost.
It is submitted that improvements in the design of magnetoresistive sensors, particularly recessed sensor devices of the type disclosed in the ""547 patent, are needed in order to overcome the aforementioned disadvantages. What is required is an magnetoresistive sensor in which flux delivered to the sensing element is not diluted, and wherein increased longitudinal biasing strength is provided.
The foregoing problems are solved and an advance in the art is obtained by a novel magnetoresistive sensor for use in a data storage device. The sensor has a sensing ferromagnetic (free layer) that is selectively pinned using exchange biasing to define longitudinally biased magnetically responsive (active) free layer regions of a sensing element and a flux guide that magnetically connects the sensing element to a sensing surface of the sensor. The sensing element may be constructed as an MTJ device, a CPP spin valve device, or in accordance with any other suitable design. The sensing element and flux guide active free layer regions are longitudinally biased by placing a layer of exchange bias material under areas of the free layer that lie outside the track width boundaries of the sensing element, thereby pinning those areas and making them non-magnetically responsive (non-active). The sensing element and the flux guide preferably share common track width boundaries so that magnetic flux directed from the flux guide into the sensing element is not diluted with consequent loss of sensitivity. To that end, the flux guide can be formed by extending the sensing element active free layer region forwardly and rearwardly of the sensing element stripe height boundaries, such that the flux guide runs from the sensing surface of the sensor to a location that is rearward of the back edge of the sensing element. The sensing element also includes an electrically conductive fixed ferromagnetic layer (pinned layer) and a barrier layer disposed on the pinned layer. These are in addition to the aforementioned active free layer region, which is disposed on the barrier layer. First and second electrically conductive leads and/or shields are respectively disposed to provide an electrical current generally perpendicularly through the planes of the sensing element layers.
The invention further contemplates a disk drive and a read head that respectively incorporate a magnetoresistive sensor as summarized above. A method for fabricating the sensor is also provided. According to the inventive method, a multi-layer wafer is formed that includes layers which will serve as the tunnel junction and the flux guide of the sensor. Common track width boundaries are defined for the sensing element and the flux guide in a single photo processing operation. As part of this operation, the portions of the free layer that comprise the sensing element and flux guide active free layer regions are completed, and the free layer non-active areas are exchanged biased outside the track width boundaries.
The wafer forming step of the inventive method preferably includes the formation of a wafer having an antiferromagnetic layer, a pinned layer, a barrier layer, a partial free layer (free sub-layer), and a cap layer. The photo processing operation includes applying a photoresist mask on the wafer to define the sensing element and flux guide track width boundaries. The cap layer, the partial free layer, the barrier layer and the pinned layer are then removed from areas thereof that lie outside of the track width boundaries to expose a corresponding area of the antiferromagnetic layer. An insulating layer is formed over the exposed area of the antiferromagnetic layer and an exchange bias layer is formed over the insulating layer. The photoresist mask and the cap layer are then removed to expose the partial free layer lying within the track width boundaries while leaving the exchange bias layer intact outside of the track width boundaries. Additional free layer material is then applied over the wafer, including the partial free layer and the exchange bias layer. This added free layer material completes the active free layer regions of the sensing element and the flux guide within the track width boundaries, and provides a pinned free layer outside of the track width boundaries that longitudinally exchange biases the active free layer regions to a selected magnetic orientation in the absence of an external magnetic field.